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METHOD FOR FABRICATING LASER TUNABLE INTEGRATED COLOR MEMORY THIN FILM POTENTIOMETER

IP.com Disclosure Number: IPCOM000008494D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-18
Document File: 3 page(s) / 158K

Publishing Venue

Motorola

Related People

E. S. Ramakrishnan: AUTHOR [+3]

Abstract

Tunable resistors are required for ICs and MMICs for such functions as feedback, isolation, self-biasing termination and voltage dividers in bias network. Conventional resistors in KS are fabricated by thermal diffusion, where the range of resistance are limited and also the accuracy with respect to designed values is less than desirable. The tempera- ture coefficient of resistance are difficult to compute because of the variation in doping density with depth. Ion implantation is used for higher values of resistance requiring better accuracy, and tolerance; this, however, requires significant capital invest- ment. We have developed an K-compatible method for fabricating a tunable thin film resistor.

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MOTOROLA Technical Developments

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METHOD FOR FABRICATING LASER TUNABLE INTEGRATED COLOR MEMORY THIN FILM POTENTIOMETER

by E. S. Ramakrishnan, Wei-Yean Howng and Kantyu lnoue

  Tunable resistors are required for ICs and MMICs for such functions as feedback, isolation, self-biasing termination and voltage dividers in bias network. Conventional resistors in KS are fabricated by thermal diffusion, where the range of resistance are limited and also the accuracy with respect to designed values is less than desirable. The tempera- ture coefficient of resistance are difficult to compute because of the variation in doping density with depth. Ion implantation is used for higher values of resistance requiring better accuracy, and tolerance; this, however, requires significant capital invest- ment. We have developed an K-compatible method for fabricating a tunable thin film resistor.

  Metals and alloys possess characteristic colors, which depend on the electronic band structure. The color is determined by the crystal structure, and determined by thermal treatment. Alloys of silver-zinc, nickel-tin, etc., show a reversible phase transforma- tion and display interesting characteristics because of their anisotropic properties. The silver-zinc (1: 1) alloy in its normal state has a silver color, but also exists in a metastable phase which is pink in color. The 'pink' phase is formed by heating and rapid cooling, whereas, the phase formed by a slow cooling after a high temperature anneal is silver in color. Retention of such 'color memory' in alloys have been known for optical memory applications, where data have been 'erased' and 'rewritten', for example by laser treatment. However, the electrical properties of these alloys have not been explored.

We have observed that the two phases display distinctly different values of electrical resistance, a

phenomenon that has not been reported before. The 'silver color' (slow cooled) phase, has a much higher sheet resistance (Rs), but the resistance drops to half the original value for the rapid cooled 'pink' phase. The significant feature is that the two phases are reversible, and the required resistance state can be selectively 'tuned' by thermal processing. The lower resistance state is stable, unless transformed by a thermal treatment to >475'C. The sheet resis- tance values, that were measured using a 4-point probe, showed a better than 2 to 1 ratio for the two phases. The closest known technology that uses a resistance change, is the Giant Magneto Resistance (GMR) films. The best values of reported resistance change in GMR films is -lo%, where the devices need to be subjected to large magnetic fields in order to achieve a resistance change.

  The Ag-Zn films of composition in the range 40-60 to 60-40 (atomic percent) were deposited on silicon and fused quartz substrates by rf magnetron sputtering in an argon atmosphere. Depending on the film composition, the 'low' resistance values were in the range 0.36-...